
#include<stdio.h>

#include<math.h>

int main(void)

{

	int denominator, flag;

	double esp, item, sum;

	scanf("%lf", &esp);

	flag=1;

	denominator=1;

	sum=0;

	item=1.0; 

	do{

		item=flag*1.0/denominator;

		sum=sum+item;

		flag=-flag;

		denominator=denominator+3;

		}while(fabs(item)>esp);

	printf("sum = %.6f\n", sum);

	

	return 0;

 } 
